The demand for wireless broadband data has consistently increased. Unlicensed spectrum (i.e., frequency spectrum that does not require a license from an appropriate regulating entity) is being considered by wireless cellular network operators to increase the capacity of existing services that are offered over licensed spectrum.
The use of unlicensed spectrum in the Third Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) system has been proposed as Licensed Assisted Access (LAA). Under LAA, the LTE standard is extended into unlicensed frequency deployments, thus enabling operators and vendors to maximally leverage the existing or planned investments in LTE hardware in the radio and core network.
Apart from the LAA operation considered in Release 13 of the 3GPP standard, LTE may also be operated via dual connectivity or the standalone LTE mode which may not require much assistance from the licensed spectrum Recently, a new LTE based technology “MuLTEfire” has been under consideration, requiring no assistance from the licensed spectrum to enable a leaner, self-contained network architecture that is suitable for neutral deployments where any deployment can service any device. The operation of LTE on the unlicensed spectrum without any assistance from licensed carrier will be referred to as standalone LTE-U herein.
One concern with LAA and standalone LTE-U is the co-existence of the LTE radio nodes and other radio access technologies (RATs), such as WiFi and/or other LAA networks deployed by other operators using other unlicensed radio nodes. To enable the co-existence of the LTE radio nodes and other unlicensed nodes, listen-before-talk (LBT) (also called Clear Channel Assessment (CCA)) has been proposed. LBT is a contention protocol in which the LTE radio node determines whether a particular frequency channel is already occupied (e.g., by a WiFi node) before using the particular frequency channel. That is, with LBT, data may only be transmitted when a channel is sensed to be idle.
In LTE, reference signals, such as Discovery Reference Signals (DRS), are transmitted to enable User Equipment (UEs) to “discover” an active channel. For example, a UE may sense the DRS to determine appropriate time and frequency compensation parameters for the channel. For licensed spectrum, the DRS may be periodically transmitted. Due to the unpredictability of LBT, however, for LTE-LAA and standalone LTE-U, periodic DRS transmissions may not be feasible.